I have Health Equity for a HSA because that's who my employer set it up with. For our plan (I don't know if this varies), you can invest any amount over $2000 - in other words, you must keep $2000 in cash. If your cash balance is a minimum of $2500, the monthly fee is waived.
There are many mutual funds to choose from, most of which have a lot of fees. There are Vanguard ones available for an additional fee - Health Equity adds a surcharge.
All my HSA investment funds are in the institutional S&P 500, with an ER of 0.02%. It was the most attractive (lowest fees) available in Health Equity. There's a fee of some percentage of invested monies too, but I don't remember how much.
I'm not thrilled with the HSA fees, but I max it out in order to get the tax advantages, so it's worthwhile.